The present invention pertains to methods and apparatus for detecting radiation, and more particularly to detection of photons in a semiconductor structure. The invention is advantageously employed in the detection of electromagnetic radiation, especially infrared and far-infrared radiation.
Currently, prevalent infrared photodetection technology is based on interband absorption in bulk mecury-cadmium-teluride (HgCdTe, MCT) typically operating at cryogenic temperatures and thereby imposing attendant cost and size requirements. Moreover, material homogeneity constraints limit the applicability of MCT in the context of fabricating large focal plane array devices.
Infrared detectors based on type-II superlattice structures engineered by deposition of a stack of successive semiconductor layers have shown promise for thermal imaging applications because of possible suppression of dark currents and Auger recombination. Superlattice detectors, based on optical transitions between respective electron and hole minibands in type-II superlattices of alternating III-V compounds, are theoretically superior to MCT at wavelengths long ward of 12 micrometers. Superlattice detectors, are described by Smith et al., Proposal for Strained Type II Superlattice Infrared Detectors, J. Appl. Phys., vol. 62, pp. 2545-48 (1987), which is incorporated herein by reference. Like MCT photodiodes, superlattice detectors are also typically limited to cryogenic operation.
Quantum well intersubband photodetectors (QWIPs) are based on absorption between subbands as a photodetection mechanism rather than absorption between the valence and conduction bands. Due to quantum selection rules, intersubband transitions cannot be photo-excited by normal-incidence radiation (i.e., radiation polarized in the plane of the absorption layer), so grating structure is often introduced for normal incidence detection, adding cost and complication of fabrication to the device structure. QWIPs are reviewed in detail in Levine, Quantum-Well Infrared Photodetectors, J. Appl. Phys., pp. R1-R81 (1993), incorporated herein by reference.
Photodetectors based on type-I intersubband quantum cascade laser (QCL) structures have also been demonstrated, as described, for example, in Hofstetter et al., Quantum-Cascade-Laser Structures as Photodetectors, Appl. Phys. Lett., vol. 81, pp. 2683-85 (2002), incorporated herein by reference. Like QWIPs, these detectors are also based on intersubband photo-excitation, and therefore, as in the case of QWIPs, quantum selection rules preclude their application to normal incidence radiation.
Various applications, particularly in the field of line-of-sight communications and thermal imaging, make high-bandwidth detection of normal-incidence infrared radiation very desirable, especially if room-temperature photovoltaic operation is achieved.